This.

While lots public domain books are on Amazon's store, most of those books are not free, both in the sense of "free or charge" and "DRM free". A lot of literature classic are released by a major publishing house with foreword and annotations, which to be fair, are copyrighted works and provide value. And they cost a bit of money. The "real" public domain versions provide by Amazon are barebone. Those versions are often good enough for many people, but you don't need to get them from Amazon in the first place.

In other words, public domain or not does not have much to do with DRM-free or even Amazon.